Active radar is an object-detection system that uses radio waves to determine the range, altitude, direction and/or speed of objects. Active radar can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations and terrain. An active radar system can include a radar dish or antenna that transmits pulses of radio waves or microwaves that bounce off any object in their path. The object reflects a small part of the wave's energy to a dish or antenna that can be located at the same or different site as the transmitter.
Passive sensor systems (also referred to as passive coherent location, passive covert radar and passive radar systems) encompass a class of radar systems that detect and track objects by processing reflections or transmissions from sources of illumination in the environment. Such sources can include, but are not limited to communications signals and commercial broadcast signals that are transmitted or reflected from a target. In some situations, the sources can be cooperative, and in other situations, the sources can be non-cooperative. The term “passive sensor system” can indicate a system that is configured to detect all such sources or some subset thereof.